The present invention relates in general to solid state voltage reference elements and, in particular, to bandgap voltage reference elements, which can operate from a low supply voltage.
Without limiting the scope of the invention, its background is described in connection with the design of bias circuits for integrated circuits using CMOS technology. It should be appreciated by one skilled in the art that the principles of the present invention may be implemented in a wide variety of applications.
A bandgap reference circuit 10 known in the prior art is shown in FIG. 1A. Bandgap reference circuit 10 generally comprises bipolar transistors 12 (Q1) and 14 (Q2), resistors 16 (R2), 18 (R1), and 20 (R1), operational amplifier 22, and an offset voltage source 24. (R1 represents the resistance of either resistor 18 or resistor 20.) Under ideal situations, the voltage provided by the offset voltage source 24 is zero volts. In addition, the base terminal and the collector terminal of bipolar transistors 12 (Q1) and 14 (Q2) are shown connected to a power supply voltage terminal 26 (VSS), which is typically zero volts.
The equation describing the bandgap output voltage VBG of the circuit 10 in FIG. 1A is shown below:
VBG=VBE+(R1/R2)xc2x7VBE,xe2x80x83xe2x80x83(1)
where VBE is the base-to-emitter voltage across bipolar transistor 12 (Q1), and xcex94VBE is the difference of the base-to-emitter voltages between bipolar transistor 12 (Q1) and bipolar transistor 14 (Q2), which are operated at different current densities. VBE has a negative temperature coefficient, while xcex94VBE has a positive temperature coefficient. The xcex94VBE voltage is imposed across resistor 16 (R2). An image of the current flowing through resistor 16 (R2), which is xcex94VBE/R2, is forced to flow through resistor 18 (R1). This gives rise to the term (R1/R2)xc2x7xcex94VBE.
By properly selecting the value of (R1/R2), the magnitude of the positive temperature coefficient term can be scaled and then added to the negative temperature coefficient term to substantially cancel the temperature effects. This zero, or extremely low, temperature coefficient output voltage is known as the bandgap output voltage.
A bandgap reference circuit can therefore provide a stable output voltage with respect to temperature. Furthermore, a bandgap reference circuit, such as the one shown in FIG. 1A, can also be designed such that when the supply voltage exceeds a minimum voltage level for proper biasing, the bandgap voltage reference will have a very good power supply rejection ratio. These characteristics makes the bandgap reference circuit a desirable candidate for use as a voltage reference for integrated circuits such as analog-to-digital converters, digital-to-analog converters, and bias current generators. In general, a bandgap voltage reference may be used in analog circuits or mixed-signal circuits to generate a stable, temperature-independent voltage reference.
It has been described in the prior art that it is very desirable to generate as large as possible of a xcex94VBE term, which is the voltage drop across resistor R2, in order to make the term (R1/R2) as small as possible. The (R1/R2) term increases any non-ideal conditions associated with the generation of xcex94VBE, as well as any noise voltage associated with R2.
In previous attempts to increase the xcex94VBE term, a stack of two VBE diode arrays has been used. FIG. 1B shows a circuit in the prior art utilizing this approach. The circuit in FIG. 1B has a diode stack containing two diodes. The voltage difference between the two stacked diodes yields a larger xcex94VBE term. This will yield a xcex94VBE term that is twice as large as the xcex94VBE term generated in the circuit 10 shown in FIG. 1A.
However, the addition of stacked diode arrays increases the minimum supply voltage required for proper operation, as well as increasing the overall physical size of the bandgap voltage reference circuit. For a sub-micron CMOS process, the number of diodes in a stack is limited to a stack of two diodes due to the maximum allowed supply voltage, which is typically 3.6 volts. Therefore, the usefulness of stacked diode arrays to minimize noise associated with the (R1/R2) term is limited.
In a sub-micron CMOS process, using a bandgap voltage reference circuit as the voltage reference of a high resolution (i.e., greater than 12 bit resolution) analog-to-digital converter or digital-to-analog converter, the noise present at the output of the prior art bandgap voltage circuit is very large and must be filtered out. This filtering is typically accomplished with a large value capacitor, which is external to the integrated circuit. Electrical connection to this external capacitor is achieved with bond wires and package leads. The problem of electromagnetic coupling to the bond wires and package leads limit the usefulness of external capacitors as filter elements. Thus, it is desirable to have a bandgap voltage reference circuit with low noise so that an external capacitor is not required.
From the foregoing, it can be appreciated that a need exists for a voltage reference circuit that overcomes the problems in the prior art. It is desirable that such a voltage reference circuit has an output voltage that is not subject to substantial variations due to temperature changes. It is further desirable that such a voltage reference circuit has a lower output noise voltage than the prior art. It is believed that the features of the present invention described herein solve and address the foregoing problems and limitations.
In accordance with the present invention, a bandgap voltage reference circuit is provided that is associated with low power supply voltage operation. The value of the power supply voltage can be as low as approximately 1.3 volts, which makes the inventive bandgap voltage reference topology suitable for sub-micron CMOS processes wherein supply voltages may typically range from approximately 1.8 volts to approximately 3.6 volts.
An improved bandgap voltage reference circuit is described herein for providing a stable reference output voltage. In accordance with a preferred embodiment of the present invention, the improved bandgap voltage reference circuit comprises a pair of bipolar transistors connected in common collector configuration and operating at different emitter current densities. The circuit further comprises resistors connected in series with each of the bipolar transistor emitters for establishing a voltage drop. The circuit further comprises a pair of CMOS transistors connected in common source configuration and functioning as current sources, wherein the source terminals are connected to a positive supply voltage, and the drain terminals are connected with the resistors. The circuit further comprises an operational amplifier wherein the output terminal is connected to the gates of the CMOS transistors.
The circuit also includes a CMOS transistor operating as a positive temperature coefficient current source, wherein the gate is connected to the output of the operational amplifier, and the source is connected to a positive voltage supply. Another CMOS transistor is included, which operates as a current source, wherein the gate is connected to the output of the operational amplifier, and the source is connected to the positive voltage supply. The circuit further comprises another bipolar transistor for serving as the output device of the bandgap voltage generator, wherein the base is connected to the drain of the CMOS transistor operating as a current source.
The circuit of the present invention may also include a base compensation circuit for canceling any errors introduced into the circuit by the base current of the output bipolar transistor. The circuit of the present invention may further include a feedback voltage adjustment circuit comprising resistive elements and switch elements controlled by digital logic.
A preferred method of providing a bandgap voltage reference is also disclosed herein. The inventive method comprises the steps of operating a pair of bipolar transistors at different emitter current densities, providing one or more resistive elements in series with the pair of bipolar transistors for establishing a voltage drop, operating a pair of CMOS transistors as current sources, configuring an operational amplifier for providing a positive temperature coefficient current source, providing a control voltage for the positive temperature coefficient current source, providing a positive temperature coefficient voltage source, and providing a third bipolar transistor as a bandgap voltage output device. The method may further comprise the step of offsetting error introduced by the base current of the bandgap voltage output device. The method may further comprise the step of adjusting the value of the resistance of the resistive elements.
For a more complete understanding of the present invention, including its features and advantages, reference is now made to the following detailed description, taken in conjunction with the accompanying drawings.